This invention relates to fail-safe control circuitry, and in particular, to circuitry that is particularly adapted for vehicle transportation control such that it must have fail-safe characteristics.
A basic requirement imposed on circuitry for controlling transportation systems is that it be fail-safe; that is to say, if a malfunction should occur, this should result in the activation of a device or the application of emergency equipment in such a way as to protect the passage of the vehicle and the security of its passengers.
Accordingly, in the design of control circuitry and, more particularly, in the design and construction of an active bandpass filter to be utilized in control circuitry or in a sub-system of a railroad control system, it is mandatory that such filter also possess fail-safe characteristics, by which is meant that it should not develop a spurious signal due to self-oscillation so as to produce unsafe effects.
Concomitantly with the objective of utilizing an active bandpass filter in certain applications is the need to construct such a filter such that it will have minimum space requirements and not require the use of inductors. Moreover, because of the small sizes made practical for filter applications, the possibility exists of replacing much bulkier devices such as decoder transformers used in detecting rate codes in certain kinds of apparatus.
In order to furnish the man skilled in the art with background material for the present invention, reference may be made to a Dec. 5, 1973 article in EDN by James J. Murphy entitled, "Derive Parallel T Current Gain Filters from Voltage-Null Network". That article describes certain modifications to a general parallel-T RC network useful as a voltage-null frequency selective network.
Accordingly, it is a primary object of the present invention to provide an active bandpass filter which has fail-safe characteristics.
Another object is to provide such a filter when it does not involve the use of inductors or the like.
A further object is to provide an active filter for control purposes that can replace much bulkier components in control systems.
The above and other objects are fulfilled by a primary feature of the present invention according to which the filter circuit comprises essentially two sections, the first of which forms a modified twin-T filter network, and the second of which includes a number of resistors which form a feedback path and attenuator. A pair of emitter follower buffers which have high input impedance and low output impedance serve to isolate the aforenoted two sections from each other. Each of these buffers includes a transistor and suitable output resistor
Other and further objects, advantages and features of the present invention will be understood by reference to the following specification in conjunction with the annexed drawing, wherein like parts have been given like numbers.